1. Field of the Invention
The present invention relates generally to a mobile station in a CDMA (Code Division Multiple Access) communication system, and in particular, to a device and method for acquiring a PN (Pseudo Noise) sequence in a mobile station.
2. Description of the Related Art
In a spread spectrum communication system, a transmitter spreads an information signal over a wide bandwidth and a receiver then despreads the spread signal to the original bandwidth. Spread spectrum techniques are generally classified into direct sequence techniques, frequency hopping techniques, time hopping techniques and hybrid techniques. A commercial CDMA system typically employs the direct sequence spread spectrum (DSSS) technique. That is, in the CDMA system, a base station (BS) multiplies an information signal by a PN sequence having a high data rate to spread the information signal before transmission, and a mobile station (MS) then multiplies a received signal by the same PN sequence as that generated in the base station to despread the received signal, thereby recovering the original information signal. To accomplish this, the mobile station performs sync acquisition to synchronize the phase of the PN sequence generated in the base station (hereinafter referred to as BS generated PN sequence) with the phase of the PN sequence generated locally in the mobile station.
In CDMA mobile communication systems, the sync acquisition of the mobile station can be divided into two operations, namely acquisition and tracking. The acquisition operation is used to acquire synchronization for the BS generated PN sequence within a threshold value, and can be further divided into two operations. First, the mobile station calculates detected energy depending on a correlation between the received BS generated PN sequence and the locally generated PN sequence, and second, the mobile station compares the calculated results with a threshold value. If the calculated result is less than the threshold value, the mobile station shifts the phase of the locally generated PN sequence and then calculates the detected energy for the new sequence. Otherwise, when the calculated result is higher than the threshold value, a starting point of the locally generated PN sequence will fall within a predetermined critical error, range and the mobile station performs fine synchronization by finely comparing the calculation result of the detected energy with the threshold value. That is, the mobile station performs the tracking operation.
Several algorithms have been proposed for performing the tracking operation. The algorithms may be classified into several methods. One such method involves adjustment of an integrating period. Further, the acquisition operation can be divided into a serial search method and a parallel search method.
FIG. 1 illustrates a serial acquisition circuit for using the serial search method in a mobile station in a CDMA mobile communication system.
Referring to FIG. 1, a multiplier 110 multiplies an input BS generated PN sequence value by a local PN sequence value generated from a local PN sequence generator 120 in a predetermined integrating period. The multiplier 110 provides the multiplication results to a correlator 130. The correlator 130 calculates detection energy depending on a correlation between the two PN sequence values. A controller 140 compares the detection energy with a threshold value. If the detection energy is less than the threshold value, the controller 140 provides a phase shift control signal to the local PN sequence generator 120 to shift a phase of the local PN sequence. As a result, the mobile station repeats the detection energy calculating operation and the comparing operation. Otherwise, when the detection energy is greater than the threshold value, the controller 140 determines that acquisition has been successfully performed. Thereafter, the mobile station performs the tracking operation.
Alternatively, in the parallel search method, a plurality of serial acquisition circuits simultaneously perform the acquisition operation, thereby reducing the time required for acquisition. The conventional parallel search scheme includes a plurality of the serial acquisition circuits shown in FIG. 1, arranged in parallel. Accordingly, the conventional parallel search scheme requires a plurality of controllers, or control procedures, in order to perform the required comparisons and determinations on the detection energies output from the required plurality of correlators. As a result, the conventional parallel search scheme includes an increased number of controllers and requires a complicated control operation for the detection energies. For example, the controller must examine whether any one of the detection energies are greater than the threshold value, out of the detection energies from the several correlators. To this end, the controller must first determine the detection energy having the highest energy value out of the detection energies to compare it with the threshold value, or must compare all the detection energies with the threshold value, thereby resulting in a complicated control operation.
CDMA mobile communication systems have developed from the IS-95 standard, which mainly provides voice service, into the IMT-2000 standard, which provides high-speed data service as well as voice service. The IMT-2000 standard aims at high-quality voice service, moving picture service, and Internet search service. In addition, as a test system for the IMT-2000 standard, a multicarrier system which transmits an information signal by distributing the information signal to a plurality of carriers is proposed. That is, the multicarrier system modulates the information signal, which has undergone direct sequence spreading with the same PN sequence, with different carriers and transmits the modulated information signals.
A base station for the multicarrier CDMA system converts an information signal into a plurality of parallel signals and multiplies the converted parallel signals by a PN sequence to spread the signals. The base station modulates the spread signals by multiplying the spread signals by the different carriers.
A PN sequence acquisition circuit for the multicarrier CDMA mobile communication system can be implemented using either the serial search scheme or the parallel search scheme. The acquisition circuit using the serial search scheme searches a single carrier receiving signal out of a plurality of carrier receiving signals. Alternatively, the acquisition circuit using the parallel search scheme simultaneously searches all the carrier receiving signals transmitted from the transmitter. The acquisition circuit having the serial search scheme is advantageous in that the hardware structure is greatly simplified. However, the acquisition circuit having the serial search scheme has a low acquisition speed for the local PN sequence. Further, when the carrier receiving signals undergoes frequency selectivity fading, the acquisition circuit may fail to perform acquisition even though the BS generated PN sequence is in sync with the local PN sequence. That is, even though the BS generated PN sequence is in sync with the local PN sequence, the acquisition circuit having the serial search scheme receives a carrier receiving signal having a poor frequency fading property transmitted from the base station, and thus fails to perform acquisition.